The ornamental design for a paintbrush, as shown and described.
FIG. 1 is a perspective view of the paint brush.
FIG. 2 is a front view thereof.
FIG. 3 is a back view thereof.
FIG. 4 is a left side view thereof.
FIG. 5 is a right side view thereof.
FIG. 6 is a bottom view thereof.
FIG. 7 is a top view thereof; and,
FIG. 8 is a partial sectional view of the paint brush illustrating an interior portion of a connecting portion of the paint brush. All other interior views of the connecting portion are identical to the view showed here in FIG. 8.
The broken lines showing a connecting pole in FIG. 8 are included for the purpose of illustrating environmental structure and form no part of the claimed design.